Modern wind turbines are either horizontal axis turbines or vertical axis turbines. Horizontal axis wind turbines dominate the market world wide. They normally have a nacelle, rotor and blades that sit on top of a tower. The nacelle consists of the generator, planetary gearing and all the control systems necessary to operate the turbine. The rotor holds the blades (usually 3) in their positions while they rotate around the main shaft in the nacelle. These wind turbines work for many years with little maintenance, however, they are very expensive. The economics of horizontal axis wind turbines have been improving, but still need subsidies in most parts of the world to be an economical energy alternative.
The vertical axis lift type wind turbines (excluding drag type turbines) such as the darrieus rotor, gyro rotors, or the H style turbines, have had moderate success. These turbines tend to have lower overall power efficiency and have little advantage over the dominant horizontal axis turbines. These turbines, however, do not need to be turned into the wind, they tend to be quieter and they have few moving parts.
Most turbines, whether horizontal or vertical axis, typically need towers to raise the turbines high above the ground surfaces where the wind velocity is much higher, and therefore, more beneficial. The towers are an expensive component and in most cases they limit the size of the turbine.
U.S. Pat. No. 4,134,707 belonging to Ewers and U.S. Pat. No. 6,857,846 belonging to Miller disclose examples of drag type turbines, rather than lift type turbines. The blades of the drag type turbines are arranged to rotate the turbines about a vertical axis by capturing wind energy on the faces of the blades to push the turbine in its rotation. In a drag type turbine, it is desired to maximize the overall size of the blades so that the blades span a maximum area within a given sweep area thereof. Use of various support arms and cables and the like to support the blades do not considerably affect the efficiency as the increased drag against rotation is partially offset by capturing more wind and due to the limited velocity of the turbine which is effectively limited to the speed of the wind. Each of the noted documents discloses multiple turbine sections stacked above one another, however in each instance a complex framework is required to support the large blades designed to capture as much wind as possible in a drag type turbine.
U.S. Pat. No. 7,156,609 belonging to Palley discloses one example of a vertical axis turbine formed of a plurality of individual blade sections which are assembled into a complex blade shape. The blades are supported at top and bottom ends by horizontal portions which provide drag against rotation without contributing to any beneficial lift forces to enhance rotation. Drag is typically of much greater concern in a lift type turbine as such turbines are most efficient when rotating at speeds which are plural times the speed of surrounding winds. Furthermore no additional structural support is provided to the blades along the length thereof which, in a lift type turbine, can be subjected to considerable centrifugal forces due to the high rotation speeds as well as strong lift forces towards the axis of rotation.
U.S. Pat. No. 5,183,386 belonging to Feldman discloses a vertical axis sail bladed wind turbine in which the blades comprise two fabric sail portions and a third cable portion arranged to be wound onto a drum in a collapsed position. The sail portion of the blades are not suitably arranged to resist strong lifting forces being generated or strong centrifugal forces from high rotation speeds and accordingly the turbine has limited application. In general the vertical portions of the sail blades would have to be quite short in order to overcome the centrifugal forces acting on them.
U.S. Pat. No. 4,624,624 belonging to Yum comprises a vertical axis turbine in which the blades are hinged for folding into a collapsed structure. Only a minimal portion of each blade is positioned at the outer periphery of the turbine where the turbine is operating at its greatest efficiency.